In recent years, the active research and development have been carried out on a nonaqueous electrolyte secondary battery, in which charge/discharge is carried out through the movement of lithium ions between a negative electrode and a positive electrode, as a high-energy density battery. In the past, a lithium ion secondary battery, which includes a positive electrode containing LiCoO2 or LiMn2O4 as an active material and a negative electrode containing a carbonaceous material that inserts and extracts lithium, has been widely put into practical use for a portable device.
In recent years, this type of lithium ion secondary battery has been widely used as an electric power source for a vehicle such as an electric vehicle or a hybrid vehicle using an engine and a motor in combination from the viewpoint of environmental problems.
When mounting lithium ion secondary battery on a vehicle such as an electric vehicle or a hybrid vehicle, a lithium ion secondary battery is required to have storage performance under a high-temperature environment, cycle characteristics, and long-term reliability on high output.
Also, it is currently difficult to mount a lithium ion secondary battery on an engine room of a motor vehicle so as to be used as a substitute for a lead storage battery. Therefore, it is required to improve the high-temperature durability (for example, 80° C. or more).
For this reason, constituent materials of a battery such as a positive electrode, a negative electrode, and a separator are required to be a material excellent in chemical and electrochemical stability, strength, and corrosion resistance.
As a material excellent in thermal stability in a high-temperature environment, lithium iron phosphate (LiFePO4), which is one of lithium phosphate metal compounds having an olivine crystal structure, is known. Lithium iron phosphate has been used as a positive electrode active material of a lithium ion secondary battery. However, lithium iron phosphate has low electron conductivity, and thus, it is not possible to obtain high output in a lithium ion secondary battery. Moreover, when using a carbon material for a negative electrode, the deterioration caused by the precipitation of metal lithium in a low-temperature environment is enhanced.
Meanwhile, as a separator for a lithium ion secondary battery, a polyolefin-based microporous polymer film (film) or a nonwoven fabric comprised of a cellulose fiber has been used. However, in a conventional separator such as a polyolefin-based microporous polymer film or a nonwoven fabric comprised of a cellulose fiber, a pinhole is easily formed when reducing the thickness. For this reason, in the lithium ion secondary battery, an internal short-circuit easily occur, and self-discharge easily occurs during high-temperature storage.
In particular, a polyolefin-based microporous polymer film has a low melting point, and thus, is thermally shrunk in a high-temperature environment (for example, 100° C.). For this reason, the positive electrode and the negative electrode may have direct contact with each other, to thereby cause internal short-circuit, and some of micropores are melted and closed, to thereby cause the decrease in output of a battery.
In this background, a separator-electrode-integrated storage device formed by integrally joining a separator to a surface of an electrode has been created. However, in spite of this technique, a lithium ion secondary battery having excellent high-temperature durability (for example, 80° C. or more) and high output has not been achieved.